The long-term goal is to understand at a detailed physical level how water, ions, and nonelectrolytes cross epithelia specialized for fluid transport. Immediate goals include the following: to characterize transport mechanisms and permeability barriers in mammalian urinary bladder; to describe in detail ion permeation mechanisms in leaky epithelia; and to estimate partition coefficients, diffusion coefficients, and interfacial resistances of solutes in model bilayers and in biological membranes, by techniques based on electron spin resonance, nuclear magnetic resonance, and radioactive tracers.